problem restoring a database in other intance of same sarver

hi hope you can help me, I will first give an overview of what I do. I am creating a script in powershell for the restare of SharePoint web applications on other farms, it is necessary to make a backup of all the elements that are to recreate or restore a new farm, as such require a new database server data. I did this allready by graphics means you could say (with the central administration and sql server), I do a web application backup, went i try to restore it I went out an error  but I solve it by restoring the .bak file directly to the new instance of sql, then created a new site collection with powershell and associate that site colletion to the restored database.
Now I want to do just that but all in a powershell script, now the problem is this, When i did this by the graphic method using 2 different sql servers with different instances, we will call the first or primary A(production), and restored for the first time B(development) . Now for power ...